text: For many applications, the comb must be widened to at least an octave: that is, the highest frequency in the spectrum must be at least twice the lowest frequency. One of three techniques may be used:
  * supercontinuum generation by strong self-phase modulation in nonlinear photonic crystal fiber or integrated waveguide
  *a Ti:sapphire laser using intracavity self-phase modulation
  *the second harmonic can be generated in a long crystal so that by consecutive sum frequency generation and difference frequency generation the spectrum of first and second harmonic widens until they overlap.
  These processes generate new frequencies on the same comb for similar reasons as discussed above.